The concept of detecting dirt in pulp or counting the number of specks, i.e. areas of different color, based on analyzing a moving web using a charge coupled device (CCD) camera has been used for example in the Intel Dirt Counter.TM. made by Intel and described in On-Line Automated Pulp Dirt Count Measurement by Kenesy et al presented at the TAPPI Proceedings Pulping Conference in 1987. This device scans the pulp surface using a CCD Camera each pixel of which functions as a discrete sensor. The camera is focused onto the pulp surface so that each pixel measured a section of pulp 0.15 mm wide. The surface of the pulp is illuminated from the same side as the camera and the dirt detected is size classified and a dirt count number weighted by particle size and normalized is determined.
U.S. Pat. No. 4,931,657 issued Jun. 5, 1990 to Houston et al also discloses a dirt counter which counts the number of pixels of intensity below a certain level as set by thresholding the signal with acceptance or rejection being based on the total area occupied by dirt particles or by the inclusion of significant large areas of single dirt spots.
It is also common practice to inspect the pulp visually and determine the quality of pulp subjectively and to base acceptance or rejection of the pulp for cleanliness on the subjective judgement of the inspector.
None of these techniques for detecting dirt are adequate or capable of defining the residual ink in recycled paper products.